Role in embryogenesis and fetal circulation
During heart development between 2 and 5 weeks, the endocardial tubes, myoepicardial plate, formation of the atria and ventricles, and the vascular system are described. The vascular system includes the venous sinus, which drains into the atrium.
In placental fetal circulation, blood from the inferior vena cava is directed by a valve through the foramen ovale into the left atrium. Blood from the superior vena cava and the coronary sinus (sinus coronarius) enters the right atrium and is directed into the right ventricle.
Next, blood flows from the right ventricle into the pulmonary trunk. Since the lungs are non-functional, blood is shunted through the ductus arteriosus (ductus arteriosus) into the aortic arch.
Phylogenesis of the venous sinus
In the venous system of vertebrates, blood reaches the venous sinus via two main pathways.
Somatic drainage:
- from the posterior part of the body, blood flows into the posterior cardinal veins;
- from the anterior part of the body, into the anterior cardinal veins;
- Cuvier's ducts are formed on each side by the merging of the anterior and posterior cardinal veins;
- the right and left ducts of Cuvier carry blood into the venous sinus.
Visceral drainage:
- blood from the intestines travels via the subintestinal vein;
- subsequently, the hepatic portal system is formed;
- blood leaves the liver via the hepatic vein and enters the venous sinus.
Vessels that open into the venous sinus and carry venous blood: right duct of Cuvier, left duct of Cuvier, hepatic vein.
In reptiles, the venous sinus is reduced. Venous blood from the posterior part of the body is collected into the inferior vena cava and enters the right atrium; from the anterior part of the body, it drains into two superior vena cavas and then into the right atrium.
Physiological significance: Stannius experiment
The role of the venous sinus in cardiac automatism is demonstrated by Stannius's first ligature.
- Application site: the sulcus between the venous sinus and the atria.
- Type: isolating ligature, tied tightly.
- Goal: separation of the sinuatrial node, the 1st-order pacemaker, from the rest of the heart chambers.
- Effect: isolation of the venous sinus from the atria and ventricle.
- Result: the atria and ventricle stop; a flat line is registered on the mechanogram. The venous sinus continues to contract at its own intrinsic rhythm.
- Physiological rationale: the experiment proves that the primary pacemaker is the venous sinus. The cessation of activity in underlying parts is due to the lower-order automaticity centers having lower lability and requiring time to "warm up," or being suppressed by the high-frequency impulses of the sinus.
Clinical significance and collateral vessels
The morphological features of the coronary system include a network of collateral vessels structurally similar to capillaries. They maintain blood flow during occlusion of the main coronary arteries.
Thebesian vessels:
- connect venules and veins directly to the heart cavities;
- through these, 20–50% of deoxygenated venous blood enters the cavity of the right ventricle and other chambers;
- they prevent the development of venous stasis even when outflow through the venous sinus is completely blocked, such as during increased pressure in the right atrium due to tricuspid stenosis.
Vierordt's vessels connect arterioles to the heart cavities.
Do not confuse with: scleral venous sinus
In anatomy, there is a similarly named structure — the scleral venous sinus (sinus venosus sclerae), or Canal of Schlemm.
It belongs to the drainage system of the eye. Aqueous humor flows out from the chambers of the eyeball through the spaces of the iridocorneal angle into the scleral venous sinus, and subsequently into the veins of the eye.
In the eye drainage system, the pathway is described as follows: spaces of Fontana → Canal of Schlemm, i.e., scleral venous sinus → scleral venous plexus.
Class and exam questions
Where does blood from the coronary sinus go in the fetus?
Blood from the coronary sinus enters the right atrium and is directed into the right ventricle.
What does Stannius's first ligature demonstrate?
It demonstrates that when the venous sinus is isolated from the atria, the atria and ventricle stop beating, whereas the venous sinus continues to contract at its own rhythm.
Which vessels open into the venous sinus in vertebrate phylogenesis?
The right and left ducts of Cuvier and the hepatic vein open into the venous sinus, carrying venous blood.
What prevents venous stasis when outflow through the venous sinus is blocked?
The Thebesian vessels connect venules and veins to the heart cavities and prevent the development of venous stasis even when outflow through the venous sinus is completely blocked.
What else to study on this topic
- Fetal circulation and its features
- The cardiac conduction system and Stannius's experiment
- Phylogeny of the vertebrate circulatory system
- Morphological features of the coronary system: Thebesian and Vieussens vessels
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